Polishing of semiconductor substrates

ABSTRACT

An aqueous polishing composition having abrasive particles of a metal oxide has a pH that undergoes pH drift, because ions are provided by dissolution of the metal oxide, and the pH drift is minimized by providing the aqueous polishing composition with an equilibrium concentration of the ions at said pH.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/734,095, filedDec. 11, 2000 U.S. Pat. No. 6,379,406 which claims the benefit ofProvisional Application No. 60/170,612 filed Dec. 14, 1999.

FIELD OF THE INVENTION

The present invention relates to high purity abrasive particlescomprising metal oxide for use in chemical-mechanical polishingcompositions (or slurries). Polishing compositions or slurries used forchemical mechanical polishing (CMP) typically comprise an aqueoussolution which contains abrasive particles, an oxidizing agent, adispersant and/or a complexing agent. The abrasive particles aretypically sub-micron in size.

RELATED ART

U.S. Pat. No. 4,959,113 discloses an aqueous polishing compositioncontaining alumina particles and aluminum nitrate that is added to theaqueous polishing composition to improve the polishing efficiency of thealumina particles.

SUMMARY OF THE INVENTION

This invention provides an aqueous polishing composition for polishingsemiconductor substrates comprising submicron particles of a metal oxideand a soluble metal salt of the metal oxide. The soluble metal salt ispresent in a proportionate amount to adjust the aqueous concentration ofmetal ions to the equilibrium solubility of the metal oxide at thedesired pH of the polishing composition thus providing an aqueouspolishing composition having a stable pH.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a solubility diagram for solid alumina.

FIG. 2 is a graph of pH versus time for an aqueous dispersion ofalpha-alumina.

FIG. 3 is a graph of pH versus time for an aqueous dispersion of aluminacomprising alpha-alumina and gamma-alumina phases.

DETAILED DESCRIPTION

In known CMP, the substrate being polished is rinsed in the slurry whilean elastomeric pad is pressed against the substrate. The pad and thesubstrate are moved relative to each other under load (also referred toas downforce) resulting in material removal from the substrate surfacedue to: friction between the pad and the substrate; mechanical action ofthe abrasive particles resulting in wear of the substrate; and chemicalinteraction of other slurry constituents with the substrate. Thus, therate of material removal from the substrate surface is determined by theapplied pressure, the velocity of pad rotation, and slurry constituents.CMP is extensively used for polishing semiconductors and semiconductordevices, particularly for manufacturing integrated circuits.

The polishing rates or material removal rates are increased by addingconstituents to the slurries which are corrosive to the substrate. Theseconstituents typically comprise chemicals that are oxidizing agents,complexing agents, acids and/or bases. The combined action of theabrasive particles and the other constituents results in high polishingrates or material removal rates. For example, in CMP applications forpolishing dielectric/metal composite structures, slurry constituentsaccelerate the wear and/or reactive dissolution of the metal portion ofthe structure. The purpose of this technique is to preferentially removethe metal portion of the structure so that the resulting surface becomescoplanar with an insulating or dielectric feature, typically composed ofsilicon dioxide. This process is termed chemical mechanicalplanarization.

Abrasive particles of the polishing composition of this inventioninclude but are not limited to metal oxides such as alumina, ceria,silica, titania, germania, zirconia or combinations thereof. These metaloxides are also combined with other inert abrasives such as diamond,silicon carbide, boron carbide, boron nitride, or combinations thereofIn an embodiment, the polishing composition of this invention containsabrasive particles that are readily dispersible in an aqueous medium andcomprise alpha-alumina and gamma-alumina in varying weight percentages,for example 25 and 75; 50 and 50; 100 and 0, respectively. The abrasiveparticles have a surface area ranging from about 40 m²/g to about 430m²/g, an aggregate size distribution less than about 1.0 micron and amean aggregate diameter less than about 0.4 micron.

Material removal rates during CMP are observed to be a function of thepH of the polishing composition. Thus, a polishing composition with astable pH value during the shelf-life of the polishing composition isnecessary for uniform material removal rates. A stable pH as definedherein is a pH value that fluctuates by less than 0.5 pH units.

Solid aluminum oxide (alumina or Al₂O₃) is available commercially asalpha-alumina, gamma-alumina and delta-alumina. These phases result fromvarious steps in the dehydration sequence of hydrated aluminum oxide.Alpha-alumina is harder than gamma-alumina and is typically used topolish harder substrates such as tungsten and platinum. Commerciallyavailable alpha-alumina contains the gamma and delta phases asimpurities. Since gamma-alumina dissolves more rapidly thanalpha-alumina, when the polishing composition contains alumina with thedominant phase being gamma-alumina, the pH of the polishing compositionchanges due to rapid dissolution of gamma-alumina causing an undesirabledrift in the pH of the polishing composition. Similarly, ifalpha-alumina is the dominant phase in the alumina abrasive, the pH ofthe polishing composition gradually drifts due to slow dissolution ofalpha-alumina.

In an embodiment, the polishing composition has a pH in a range of about1.5 to 5 and contains abrasive particles comprising alumina with varyingpercentages of alpha-alumina and gamma-alumina. It is found that thetime to attain a stable equilibrium pH value is a function of the weightpercent of alpha-alumina and gamma-alumina in the abrasive. Thus, inpolishing compositions containing alumina with varying percentages ofalpha-alumina and gamma-alumina, pH stability is ensured through theaddition of aluminum ions at molar concentrations up to 10 M. In anembodiment of this invention, dissolved aluminum(III) or Al³⁺ ions areprovided at an initial concentration of 1M to obtain a polishingcomposition with a stable pH value of about 2. The molar concentrationof dissolved aluminum ions at a particular pH is determined fromsolubility diagrams of alumina at various pH values, as shown in FIG. 1.FIG. 1 is extracted from “FIG. 4—Influence of pH on the solubility ofAl₂O₃ and its hydrates, at 25 C,” in the Aluminum Section of the Atlasof Electrochemical Equilibria, Marcel Pourbaix, 1966.

A polishing composition with an immediately stable pH can be obtained byadding a soluble aluminum salt, for example, aluminum chloride, aluminumnitrate, aluminum sulfate, aluminum citrate etc. to a polishingcomposition containing alumina as the abrasive. The desired stable pH isattained by adjusting the concentration of the soluble aluminum saltsuch that the Al³⁺ concentration corresponds to the equilibriumsolubility value of alpha-alumina shown on the Pourbaix diagram, FIG. 1.FIG. 1 is extracted from “FIG. 4—Influence of pH on the solubility ofAl₂O₃ and its hydrates, at 25 C,” in the Aluminum Section of the Atlasof Electrochemical Equilibria, Marcel Pourbaix, 1966.

For example, gamma-alumina and/or alpha-alumina will undergo thefollowing dissolution reaction at any pH below about 5.

Al₂O₃+3H₂O=2Al³⁺+6OH⁻

The generation of OH⁻ ions during this dissolution reaction causes thepH to drift to higher values. Thus, in a polishing composition with a pHof 2, the presence of alpha-alumina and/or gamma-alumina will cause thepH to drift upward to the point where an equilibrium is reached betweenthe pH and the amount of Al³⁺ ions in solution.

At a pH greater than 5, the following reaction needs to be inequilibrium.

Al₂O₃+H₂O=2AlO₂ ⁻+2H⁺

The generation of H⁺ ions during this dissolution reaction ofalpha-alumina and/or gamma-alumina causes the pH to drift to lowervalues.

The Figure discloses graphs representing solubility versus pH fordifferent materials. The graphs, indicated at a, b, c, d and e,represent solubility versus pH for the materials, Hydrargillite,Bayerite, Bohmite, Al₂O₃ and Al(OH)₃, respectively. Each of the graphshas a corresponding apex at about 5 pH. A broken line 1′ indicates thateach apex occurs at about 5 pH. The graph, e, has branches, indicated at3 and 4, respectively, extending from a corresponding apex.

As indicated by FIG. 1, as the pH moves further away from a value of 5,the amount of Al³⁺ ions needed to be in equilibrium with alumina orAl₂O₃ increases logarithmically. The pH drift stops once the systemreaches equilibrium between the amount of dissolved aluminum ions oraluminate ions and the amount of acid or base in the solution,respectively. The time to equilibrium is short (order of hours) withpure gamma-alumina (due to the higher solubility rate of gamma-alumina)or more than a month where the alumina comprises primarilyalpha-alumina. The time to equilibrium increases in proportion to theweight percentage of alpha-alumina in the alumina abrasive particles dueto the slow dissolution rate of alpha-alumina.

In an embodiment, the pH-stable polishing composition according to thisinvention also contains an oxidizing agent. Useful oxidizing agentsinclude any water-soluble composition capable of receiving an electronfrom the metal atoms at the surface of the substrate during thepolishing operation. By receiving electrons from the metal surface ofthe substrate, the oxidizing agent can transform metal atoms at thesubstrate surface into water soluble cations. Thus, the oxidizing agentpromotes metal dissolution. Typical oxidizing agents include: nitrates,sulfates, persulfates, iodates, periodates, peroxides and/or acidderivatives thereof. Oxidizing agents when added to polishingcompositions used for CMP convert the metal in the substrate surface toan oxide which is then subject to the chemical and mechanical action ofthe polishing composition. A general review of this art is provided byF. B. Kaufman et al., “Chemical-Mechanical Polishing for FabricatingPatterned W Metal Features as Chip Interconnects,” J. Electrochem. Soc.,Vol. 138, No. 11, November 1991, pp. 3460-3464, 1991.

In an embodiment, polishing compositions according to the presentinvention also comprise complexing agents which include compounds havingat least two acid moieties present in the structure which can effectcomplexation with the target metal in the substrate being polished. Acidmoieties are defined as those functional groups having a dissociableproton. These include, but are not limited to carboxyl, hydroxyl,sulfonic and phosphonic groups. In an embodiment, the complexing agenthas two or more carboxyl groups with hydroxyl groups in an alphaposition, such as straight chain mono- and di-carboxylic acids and saltsthereof including, for example, malic acid, tartaric acid and gluconicacid and their respective salts. Tri- and polycarboxylic acids and saltsthereof with secondary or tertiary hydroxyl groups in an alpha positionrelative to a carboxylic group are also used, for example citric acidand its salts. Examples of other compounds include ortho, di- andpolyhydroxybenzoic acid and their salts such as phthalic acid andphthalates, pyrocatechol, pyrogallol, gallic acid and gallates, tannicacid and tannates. In an embodiment, the complexing agent used in thepolishing composition of this invention tends to complex with metalanions, forming a 5 or 6 member ring, whereby the metal atom forms aportion of the ring.

A method is also provided for chemical-mechanical polishing ofsemiconductor substrates with the polishing composition of thisinvention comprising: water, high purity submicron metal oxide particlesand a soluble salt of the metal oxide present at a concentrationcorresponding to the equilibrium solubility of the metal oxide at the pHof the polishing composition. In chemical-mechanical polishing ofsemiconductor substrates, the substrate is pressed against a polishingpad and a polishing slurry is provided at the interface between thesubstrate and the pad while the pad and the substrate are moved relativeto each other under pressure. Polishing pressure or downforce controlsthe polishing rate or the material removal rate. A higher downforceresults in a faster polishing rate while a lower downforce yields apolished surface of better quality since the abrasive particles in theslurry do not scratch the substrate surface to the same extent at lowerdownforce values as at higher downforce values. During CMP, thesubstrate (for e.g. glass disks, semiconductor wafers, multi-chipmodules or printed circuit boards) to be polished is mounted on acarrier or polishing head of the polishing apparatus. The exposedsurface of the substrate is placed against a rotating polishing pad. Thecarrier head provides a controllable pressure (or downforce), on thesubstrate to push it against the polishing pad. A polishing compositionwith abrasive particles is then dispensed at the interface of thesubstrate and the polishing pad to enhance removal of the target layer(for e.g., metal in metal CMP processes). Typical downforce values arein a range of about 0.7 kPa to about 70 kPa.

EXAMPLE 1

This example illustrates the long time to equilibrium of a polishingcomposition containing alumina, comprising primarily of alpha-alumina,as the abrasive.

Alpha-phase alumina was mixed with deionized water to obtain an aqueousdispersion with an alumina content of 5%. A dispersant was then added toensure that the particles did not agglomerate. The pH of the dispersionwas then adjusted to a value of about 2.0 using nitric acid andmonitored on a regular basis for a period of 3 weeks. As shown in FIG.2, the pH of the dispersion increased from an initial pH of 2.0 to 3.7,during the observation period of about 40 days.

EXAMPLE 2

This example illustrates the effect of addition of Al³⁺ ions to apolishing composition to obtain a stable pH. Aluminum chloride was usedto adjust the concentration of Al³⁺ ions in the aqueous phase.

A polishing composition according to this invention was formulated withalumina containing both gamma-alumina and alpha-alumina. All otherconditions were identical to those of Example 1. Since gamma-alumina ismore soluble than alpha-alumina, the pH drift observed during thisexperiment was more noticeable than in Example 1, where the abrasiveparticles contained only alpha-alumina. As shown in FIG. 3, when theslurry contains an equilibrium concentration of Al³⁺ ions (correspondingto the solubility of gamma-alumina at the initial pH of 2), the pH ofthe polishing composition is stable.

EXAMPLE 3

Most other inorganic oxides such as silica, titania, and zirconia behavelike alumina in aqueous solution. For example, silica (SiO₂) dissolvesat a pH above 9. Addition of a silicate salt in a composition containingsilica as the abrasive will control dissolution of the abrasive and anyassociated pH drift.

What is claimed is:
 1. In a method of controlling pH of an aqueouspolishing composition having abrasive particles of a metal oxide thatprovides ions upon dissolution, the improvement comprising the step of:providing the aqueous polishing composition with an equilibriumconcentration of said ions at said pH, which avoids drift of said pHcaused by dissolution of the particles.
 2. In the method as recited inclaim 1, the improvement further comprising the step of: providing saidions as metal ions with said pH of
 5. 3. In the method as recited inclaim 1, the improvement further comprising the step of: providing saidions as metal ions with said pH in a range of 1.5 to
 5. 4. In the methodas recited in claim 1, the improvement further comprising the step of:providing said ions as metal ions with said pH of 5, and the abrasiveparticles comprising alumina.
 5. In the method as recited in claim 1,the improvement further comprising the step of: providing the aqueouspolishing composition with a soluble salt providing said equilibriumconcentration of said ions at said pH.